1. Field of the Invention
The invention relates to a method of adjusting a receiver of signals, corresponding to digital data packets which are transmitted in bursts, in a communications system. This method is intended to permit dynamical adaptation of the receiver to the characteristics of the various bursts which are successively received, in the case where at least one communication involves a transmission carried out by a succession of bursts.
The invention also relates to receivers that comprise a device allowing the utilization of this method, during the reception of signals as envisaged hereinabove.
2. Related Art
Good reception of signals transmitted by radio conventionally involves tuning the receiver to the transmission frequency Ftx used by the apparatus transmitting the signals and gain adjustment which allows the receiver to pick up under good conditions the signals transmitted from the transmitter apparatus utilizing this frequency Ftx. High accuracy as regards frequency tuning and a wide dynamic power range are usually necessary at the receiver level. This is the case in particular when receivers receive signals, relating to different applications, that are transmitted within the context of radiocommunications systems for which common standards are fixed and where these standards are applied by hardware that is not necessarily identical. Such hardware may exhibit different characteristics, in particular as regards power, and moreover it might not be placed under the same conditions as regards transmission/reception with respect to one another.
Good adjustment of a receiver may be tricky to achieve, in particular in the case where the signals received correspond to digital data packets that are transmitted in bursts from one or possibly more transmitters. In systems where the signals are transmitted in bursts, it must be possible for a receiver to be locked quickly onto the frequency used by a transmitter, when the latter sends it information in bursts. It must also be possible for such a receiver to adjust itself quickly in terms of power so as to be able to correctly pick up any burst intended for it so as to place itself under conditions which allow it to recover the data packets constituting the payload transmitted by the bursts. Such are in particular the requirements for receivers of communication systems implementing digital modulation, of OFDM type, according to which a signal to be transmitted is split over several narrowband channels having different frequencies.
Each receiver must, in particular, be capable of discerning without fail the arrival of a burst and consequently that of the preamble which appears at the head of this burst. It must therefore be capable of tuning itself quickly to the frequency Ftx utilized for the transmission of the burst of packets and/or of tailoring its gain as a function of the power of the signal that it receives.
Conventionally, the radio signal picked up by a receiver is taken into account by a subassembly of analog means designed to permit the extraction of the radio signals that served in the transmission of the data packets destined for it and that are received by the receiver in the environment wherein it is situated.
The above-envisaged subassembly of means commonly comprises an adjustable frequency changer, acting by downconversion and an automatic gain controller. The changer is driven by a signal of frequency Frx and it is utilized in order to lower the frequency Ftx of the radio signal received. The automatic gain controller comprises an adjustable-gain amplifier which is placed downstream of the changer and whose gain is controlled by way of an adjustment signal G. Such a subassembly is designed to supply a transposed signal whose frequency is in the allowable frequency range by a first stage of the signal processing subassembly, which follows it, and whose power is matched with that allowed at input by this first stage.
Customarily, the means of the first subassembly are preadjustable so as to be effective as soon as a burst liable to contain useful data is received. It must therefore be possible if necessary to reinitialize them for each new burst in the case of a burst-based communication system.
Customarily, a receiver is furnished with two fixed reinitialization parameters that correspond, one to a predetermined value of frequency correction Frst and the other to a predetermined value of gain correction Grst. These parameters are intended respectively to be utilized so as to act, one on the gain adjustment amplifier and the other on the frequency changer.
As soon as the preamble of a new burst is received, it is processed, at the level of the receiver that receives it. The object of the processing of this preamble is to make it possible to quickly determine the signal frequency and power relating to the burst in which it is included and to define the corresponding adjustment parameters. These parameters are utilized to make it possible to act immediately on the changer and/or on the adjustment amplifier, when there is a requirement therefor, so as to allow optimum reception of the packets constituting the payload transmitted by the burst. As soon as the burst has been received in full, the changer and the adjustment amplifier are ordinarily reinitialized.
The frequency and power determinations carried out by taking account of a burst preamble may however prove to be insufficiently precise and lead to insufficiently accurate adjustments. It is possible that an incorrect frequently adjustment and/or gain adjustment may lead to a significant decrease in the reception performance of a receiver. It is also possible that a or a string of bursts transmitted may not be processed by a receiver, if the preamble detection phase is not carried out in a satisfactory manner by this incorrectly adjusted receiver.